Cleanroom Air Supply and Exhaust
Cleanroom needs enough air supply and usually at a controlled temperature and humidity. This means that in most facilities the cleanroom Air Handling Units (AHU) consume most of all the site power. As a general rule of thumb, the cleaner the cleanroom needs to be, the more air it will need to use and more air change times. To reduce the expense of modifying the ambient temperature or humidity, AHU or other systems are designed to recirculate (if product characteristics permit) about 80% air through the room, removing particulate contamination as is it generated and whilst keeping the temperature and humidity stable.
Particles (contamination) in the air tend to either float around or settle on the surface of equipment. Most airborne particles will slowly settle, with the settling rate dependent on their size.
A well-designed air handling system should deliver both “fresh” and “recirculated” filtered clean air into the cleanroom in such a way and at a rate so that it flushes the particles from the room. Depending on the nature of the operations, the air taken out of the room is usually recirculated through the air handling system where filters remove the particulates. High levels of moisture, noxious vapours or gases from processes, raw materials or products cannot be recirculated back into the room, so the air in these cleanrooms is exhausted to atmosphere and then 100% fresh air is introduced into the facility.
Rooms occasionally experience high levels of airborne particulates during routine operation, such as in a sampling room or dispensary. In these cases, the room needs to be cleaned quickly between operations to prevent cross-contamination.
The volume of air introduced into a cleanroom is tightly controlled and so is the volume of air that is removed. Most cleanrooms are operated at a higher pressure to the atmosphere, which is achieved by hiving a higher supply volume of air into the cleanroom than the supply of air being removed from the room. The higher pressure then causes air to leak out under the door or through the tiny cracks or gaps that are inevitably in any cleanroom.
A good air handling system makes sure that air is kept moving throughout the cleanroom. The key to good cleanroom design is the appropriate location of where the air is brought in (supply) and taken out (exhaust).